Concentration and association of minor and trace elements in Mukah coal from Sarawak, Malaysia, with emphasis on the potentially hazardous trace elements

International Journal of Coal Geology 88 (2011) 179–193

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Concentration and association of minor and trace elements in Mukah coal from Sarawak, Malaysia, with emphasis on the potentially hazardous trace elements Say-Gee Sia ⁎, Wan Hasiah Abdullah Geology Department, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 1 June 2011 Received in revised form 17 September 2011 Accepted 18 September 2011 Available online 2 October 2011 Keywords: Coal Health and environmental concerns Mixed layer clays Pyrite Trace elements

a b s t r a c t The ash yield and concentrations of twenty-four minor and trace elements, including twelve potentially hazardous trace elements were determined in Mukah coal from Sarawak, Malaysia. Comparisons made to the Clarke values show that Mukah coal is depleted in Ag, Ba, Be, Cd, Co, Mn, Ni, Se, U, and V. On the other hand, it is enriched in As, Cr, Cu, Pb, Sb, Th, and Zn. Among the trace elements studied, V and Ba are associated predominantly with the clay minerals. Manganese, Cr, Cu, Th, and Ni are mostly bound within the aluminosilicate, sulphide and/or carbonate minerals in varying proportions, though a portion of these elements are also organically bound. Arsenic, Pb and Sb are mostly organically bound, though some of these elements are also associated with the sulphide minerals. Zinc is associated with both the organic and inorganic contents of the coal. Among the potentially hazardous trace elements, Be, Cd, Co, Mn, Ni, Se, and U may be of little or no health and environmental concerns, whereas As, Cr, Pb, Sb and Th require further examination for their potential health and environmental concerns. Of particular concern are the elements As, Pb and Sb, which are mostly organically bound and hence cannot be removed by physical cleaning technologies. They escape during coal combustion, either released as vapours to the atmosphere or are adsorbed onto the fine fly ash particles. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Coal is a complex combustible rock made up of organic and inorganic components. By definition, coal may contain up to 50% by weight and 70% by volume of non-carbonaceous materials (Schopf, 1956), with as many as 76 of the 92 naturally occurring elements of the periodic table having been detected (Schweinfurth, 2003). The concentration of elements in coal depends on the properties of the country rocks, depositional environments, diagenesis and coalification processes, syngenetic and epigenetic mineralisation, as well as hydrological/hydrogeological conditions (Arbuzov et al., 2011; Clarke and Sloss, 1992; Cohen et al., 1984; Dai et al., 2011a; Filippidis et al., 1996; Finkelman, 1993; Gluskoter et al., 1977; Goodarzi, 1995; Goodarzi and Swaine, 1994; Gürdal, 2011; Mukhopadhyay et al., 1998; Sun et al., 2010; Swaine, 1990; Valković, 1983). As such, the concentration of the elements can differ greatly between and within coal seams and coalfields. Based on their concentration, the elements contained in coal can generally be classified into three groups: (1) major elements, generally with concentrations higher than 1000 ppm, consisting of the coal forming elements (C, H, O, and N) and S; (2) minor elements, generally with concentration between 100 and 1000 ppm, comprising the mineral matter forming elements (Si, Al, Ca, Mg, K, Na, Fe, Mn and Ti) and halogens (F, Cl, Br, I); and (3) trace

⁎ Corresponding author. Tel.: + 60 135533931; fax: + 60 379675149. E-mail address: [email protected] (S.G. Sia). 0166-5162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2011.09.011

elements, generally with concentrations below 100 ppm (Schweinfurth, 2003; Xu et al., 2003). Elements in coal can either organically associated in coal's pore water or bound organically with coal macerals, or are inorganically bound with discrete minerals (Ward, 2002). The modes of occurrence of trace elements vary greatly among coals. In low-rank coals such as subbituminous B of Mukah coal (Sia and Dorani, unpublished), the elements are usually organically bound, but with the progress of coalification, the elements are removed by expulsion of moisture and changes in the chemical structure of the organic matter (Benson and Holm, 1985; Given and Spackman, 1978; Kiss and King, 1977; 1979; Li et al., 2007; Miller and Given, 1986; Ward, 1991; 1992; 2002). Elements which are bound with discrete minerals will, however, remain unchanged (Ward, 2002). Statistical analyses, particularly Pearson correlation coefficients and cluster analysis, have been widely applied to study the modes of occurrence of trace elements (Brownfield et al., 2005; Dai et al., 2005; 2007; 2008a; Du et al., 2009; Eskenazy and Stefanova, 2007; Golab and Carr, 2004; Hu et al., 2006; Karayigit et al., 2000; Kimura, 1998; Mukherjee and Srivastava, 2005; Pentari et al., 2004; Shaver et al., 2006; Spears and Tewalt, 2009; Spears and Zheng, 1999; Suárez-Ruiz et al., 2006; Sun et al., 2010; Wang et al., 2008; Wang et al., 2011). Statistical relationships between an element with the ash yield and ash-associated elements are frequently used to explore the organic and inorganic affinities of an element (Karayigit et al., 2000; Spears and Tewalt, 2009; Spears and Zheng, 1999; Wang et al., 2008). Elements displaying significant positive relationship with ash yield and ash-associated elements are inorganically associated, and vice versa. Negatively correlated

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elements are believed to be mostly organically associated, though a portion of the elements are also inorganically associated, and vice versa. In a regression plot between an element (y) and the ash yield (x), the intercept of the regression line on the ordinate reveals the fraction of an element that is organically associated. Minerals in coal can be grouped into four main categories, viz. aluminosilicate minerals (clay minerals), sulphide and sulphate minerals, carbonate minerals, and silica minerals (principally quartz) (Speight, 1994; 2005). Although quartz is a common mineral in coal, it usually hosts negligible amounts of trace elements (Davidson and Clarke, 1996; Finkelman and Bunnell, 2003), and sulphate minerals are only common in a weathered coal upon oxidation of pyrite (Speight, 2005). As such, the trace elements in coal are usually associated with only three major mineral groups, viz. the aluminosilicates, carbonates, and sulphides (Wang et al., 2008). Aluminosilicate minerals are the most common inorganic constituents in coals, and are present mainly as kaolinite [Al2Si2O5(OH)4], illite [K1.5Al4 (Si6.5Al1.5)O20(OH)4] and, to a lesser extent, as smectite [(Na, Ca)0.33(Al, Mg)2(Si4O10)] or illite/smectite series mixed layer clays. Kaolinite contains few trace elements, these being usually associated with illite and mixed layer clays (Finkelman and Bunnell, 2003; Spears and Zheng, 1999; Wang et al., 2008). Since K is the host element for illite/mixed layer clays, K is always used as an indicator for clay minerals in any statistical study of the modes of occurrence of trace elements. Carbonate minerals in coal are present mainly as calcite (CaCO3) and dolomite [CaMg(CO3)2], and Ca is generally used to indicate the presence of carbonate minerals in coal (Spears and Zheng, 1999; Wang et al., 2008). Although carbonate-rich coal usually contains a high Ca concentration; Ca in low-rank coals may be dominantly carboxyl-bound Ca rather than carbonate (Wang et al., 2008). Manganese, Sr, Ba, Pb, Co, and Zn are the common trace elements with carbonate affinity. Manganese may substitute for Ca in the calcite forming rhodochrosite [MnCO3], for Mg in the dolomite forming kutnohorite [CaMn(CO3)], for Fe in the siderite forming oligonite [(Fe, Mn)CO3] or rhodochrosite and for Mg in the ankerite, forming a series of solid solutions (Deer et al., 1985). A small amount of Sr, Ba, Co, and Zn may also partially replace Ca in the calcite forming strontianite [SrCO3], witherite [BaCO3], sphaerocobaltite [CoCO3], and smithsonite [ZnCO3], respectively. Zinc and Pb can commonly replace Mg forming smithsonite and cerussite [PbCO3] in dolomite (Deer et al., 1985). Sulphide minerals in coal are present as pyrite (FeS2) and marcasite (FeS2) with Fe as the indicator for sulphide (Karayigit et al., 2000; Spears and Tewalt, 2009; Spears and Zheng, 1999; Wang et al., 2008). The distribution and concentration of pyrite are of particular interest because of the potential substitution of Fe in pyrites by trace elements, e.g., Cu in chalcopyrite [CuFeS2], Pb in galena [PbS], As in arsenopyrite [FeAsS], Zn in sphalerite [ZnS], and Mn in hauerite [MnS2] forming a series of solid solutions (Deer et al., 1985). The combustion of coal is known to generate emissions of potentially hazardous trace elements (PHTEs), which may cause a wide range of health problems, especially when the coal is used indoors. This hazard also affects people living within the reach of emissions from coal-fired power stations if the latter are not equipped with emission control devices (Karayigit et al., 2000). The United States Environmental Protection Agency (US EPA) has identified the following fifteen trace elements as PHTEs that may be released during the combustion of coal: As, Be, Cd, Cl, Co, Cr, F, Hg, Mn, Ni, Pb, Sb, Se, Th, and U (US Public Law, 1990). All over the world, millions of people suffer from fluorosis, and thousands from arsenism caused by the release of the above-mentioned elements (Dai et al., 2011a; Finkelman and Bunnell, 2003; Karayigit et al., 2000). Environmental and health impacts of trace elements are generally related to the concentration, toxicity, and mode of occurrence of these elements in coals (Finkelman, 1995). The inorganically associated trace elements can be removed to a large extent applying physical cleaning technologies; this is not, however, applicable for the organically associated trace elements. The trace elements are also variable in their behaviour

during coal combustion. Depending on how these elements occur in coal, different health and environmental impacts will ensue (Finkelman and Gross, 1999; Finkelman et al., 1999; Huggins, 2002; Tewalt et al., 2001). The organically associated trace elements tend to be vaporised, either escaping to the atmosphere or are adsorbed on the fine fly ash particles upon combustion in the furnace. The inorganically associated elements are generally non-volatile or have very low volatility, and tend to be retained in the bottom ash and in the fly ash particles upon combustion (Dai et al., 2010b; Finkelman, 1994; Huang et al., 2004; Liu et al., 2004; 2005; Querol et al., 1995; Singh et al., 2011; Spears and Zheng, 1999; Vassilev and Braekman-Danheux, 1999). In this study, we determined the concentration and modes of occurrence of the minor and trace elements contained in Mukah coal, particularly the PHTEs, with the aim of assessing the environmental and health impacts of its use. This will facilitate the use of Mukah coal in an environmentally-friendly manner through (a) selective mining to avoid coal beds with high concentrations of PHTEs, (b) coal cleaning processes to remove the inorganically associated PHTEs, and/or (c) the installation of emission control devices. 2. Geological setting The Mukah coalfield is located in the low-lying coastal plain between the Mukah and the Balingian Rivers covering an area of about 300 km 2 in Sarawak. The coalfield is underlain by the Balingian Formation of Late Miocene age, which is in turn unconformably overlain by the Begrih Formation of Early Pliocene age. The contact between these two formations is marked by a wedge of basal conglomerate, called Begrih Conglomerate (Wolfenden, 1960). The coalfield has been folded into an east–west trending, westwards plunging Teres–Bakau Anticline and Bedengan Syncline (Fig. 1). The bedding dips ranges from 3° to 36°, but are commonly between 8 and 12° (Sia and Dorani, unpublished). Although there is no large scale faulting, local faults with minor displacements are common. The Balingian Formation is composed of mudstones, shales, siltstones, sandstones, tuffs, and coals. Foraminifera identified by Visser and Crew (unpublished), such as Ammodiscus sp., Glomospira sp., Haplophragmoides sp., and Trochammina sp., suggest a brackish-water depositional environment. The coalfield contains 12 coal seams, comprising 5 major (welldeveloped, with economic potential) and 7 minor seams (Fig. 2). The coal seams are usually between 1 and 2 m thick, with a cumulative coal thickness up to 16 m. Based on the 5 potentially minable major seams, the total coal resources of 551.9 Mt have been identified, comprising 20.0 Mt measured, 80.8 Mt indicated, and 451.1 Mt inferred resources (Sia and Dorani, unpublished). The coal is characterised by a high amount of vitrinite (88.8–99.1%), low to moderate amounts of liptinite (0.3–9.2%), and trace to low amounts of inertinite (b4.6%), on a mineral matter-free basis. The coal is of subbituminous B rank, with high total moisture, and low total sulphur content, and ash yield (Sia and Dorani, unpublished). Coal mining started in the Mukah coalfield in 2003. Coal extracted from this field is transported to the Sejingkat Coal-fired Power Plant in Kuching for power generation. Begrih Formation consists of conglomerates, mudstones, shales, loose sandstones, tuffs, and a thin coal seam at the base of the formation. This formation is semi-consolidated, and contains a mixed marine and brackish-water fauna in the south. However, towards the north and east, pure marine fossil such as Ammobaculites sp., Bolivia spp., Cibicides sp., Elphidium sp., Frondicularia sp., Rotalia spp., Textularia sp., and Triloculina sp., replaces the mixed fauna, which indicates an increasing marine influence (Visser and Crew, unpublished). 3. Material and methods Twenty-seven samples were picked up from eleven sampling sites (Fig. 1), applying the bench-by-bench channel sampling method. The sampling interval was decided on the basis of changes in coal lithotypes, and each sample represented a single bench sample. Information

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181

Fig. 1. Geological map of the Mukah coalfield.

regarding the sampling points is listed in Table 1. Four coal mines were visited, with sampling carried out at five mine faces (Fig. 1). Six coal outcrops exposed along the new road cuts were also studied and sampled. Latitude and longitude coordinates of the sampling points were obtained by global positioning satellite (GPS) during sample collection. Stratigraphic levels of the respective sampling points are shown in Fig. 2. Ash yield and concentrations of twenty-four minor and trace elements, including twelve PHTEs were determined. The twelve PHTEs analysed were As, Be, Cd, Co, Cr, Mn, Ni, Pb, Sb, Se, Th, and U. The ash yield of the samples was determined by a thermogravimetric method using the Perkin Elmer Diamond TG-DTA, Thermogravimetric-Differential Thermal Analyzer (ASTM D5142, 2009). Total S content was determined using the Multi-matrix analyzer, Multi EA 2000 of Analytik Jena AG, TIC-fest Solids Unit. Three selected samples were determined for their sulphate and pyritic S contents. Sulphate S was determined by treating the sample with dilute hydrochloric acid. Following this, pyritic S was determined by treating the sample with dilute nitric acid. Organic S was calculated from the total S concentration after subtracting sulphate S and pyritic S. Concentrations of other elements were determined using the Perkin-Elmer inductively coupled plasma optical emission spectrometer (ICP-OES), model Optima 5300 DV. Analytes for the analysis were prepared following the two-step digestion method using the Anton Paar Multiwave 3000 microwave digestion system. The median instead of the mean is used in this study to compare the concentration of an analysed element with its Clarke value for the world low-rank coals. This is to avoid the distortion effect of outliers and

extreme values that might be present. In the present study, the median was assigned with the assumption that all analysis results with concentrations below the detection limit were much lower than the minimum detected concentration. In the statistical analysis, a zero reading was assigned to concentrations below the detection limit. The analytical data of the elements are presented using box-plots. The lower and upper box boundaries mark the 25th and 75th percentiles of each distribution respectively, whereas the heavy black line inside the box indicates the median of the distribution. The lower and upper whiskers indicate respectively the minimum and maximum values that are not statistical outliers. The outlier marked with symbol ‘o’, is the value that is 1.5 interquartile ranges away from the 25th and 75th percentiles, while the extreme value (marked with symbol ‘*’) is the value that is 3 interquartile ranges away from the 25th and 75th percentiles. The world average of the contents for S and minor elements, as well as the Clarke values of trace elements for low-rank coals are also marked in the box-plots for references. The enrichment factors of trace elements for Mukah coal were also calculated. This factor measures the extent of enrichment by the ratio of the various trace elements in coal as compared with their respective Clarke values. If the enrichment factor is higher or lower than 1, the trace element is termed ‘enriched’ or ‘depleted’ respectively. Modes of occurrence of the elements were explored using the Pearson correlation. Interception of the elements against the ash yield plot was calculated for elements which show significant relationships with the ash yield. Cluster analysis was performed using hierarchical algorithms

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S.G. Sia, W.H. Abdullah / International Journal of Coal Geology 88 (2011) 179–193 Table 1 Sampling points information on the origin, latitude, longitude, sample number, bench thickness, and total thickness of the coal bed. Sampling Origin point

Latitude

Longitude

Sample no.

Coal bed Bench thickness thickness (m) (m)

M01

Mine face

N2° 53.781′

E112° 21.579′

M02

Mine face

N2° 51.691′

E112° 19.040′

M03

Mine face

N2° 48.874′

E112° 19.221′

M04

Mine face

N2° 50.020′

E112° 19.323′

M05

Mine face

N2° 53.714′

E112° 21.603′

037

Coal outcrop

N2° 48.301′

E112° 20.315′

038

Coal outcrop Coal outcrop

N2° 48.142′ N2° 47.953′

E112° 20.329′ E112° 20.391′

Coal outcrop Coal outcrop Coal outcrop

N2° 47.620′ N2° 51.368′ N2° 52.511′

E112° 20.835′ E112° 20.152′ E112° 20.464′

M01/2 M01/1P M01/1 M02/3 M02/2 M02/1 M03/3 M03/2 M03/1 M04/3 M04/2 M04/1 M05/3 M05/2 M05/1 037/3 037/2 037/1 038/2 038/1 039/3 039/2 039/1 043C 043B 045

0.89 0.07 0.29 0.15 0.24 0.75 0.43 0.11 1.02 0.43 0.33 1.01 0.89 0.34 0.52 0.28 0.29 0.69 0.28 0.59 0.38 0.15 1.46 0.79 0.28 0.63

0.63

046B

0.89

0.89

039

043 045 046

Fig. 2. Generalised stratigraphic column of the study area showing the stratigraphic level of the coal beds (major seam in bold).

clustering average linkage (between groups) based on the Pearson correlation coefficients of ash yield and concentration of the elements. The analysis was conducted only on elements where at least 50% of the samples recorded concentrations above the detection limit. All the analyses were performed using the SPSS statistical programme (version 16.0). Results inferred from the statistical analyses were verified using the Zeiss Supra 40VP scanning electron microscope equipped with energydispersive X-ray (EDX) and back-scattered electron (BSE) detectors, and X-ray Powder Diffraction (XRD) accessories on the low temperature ash that was prepared following the USGS ashing method (Kolker et al., 2003). Identification of the mineral grains under the scanning electron microscope was based mainly on the chemical composition of the key mineral forming elements determined coupled with the morphology of the mineral grains. X-ray Powder Diffraction (XRD) study was performed on the randomly-oriented powder of low-temperature ash of coal and carbonaceous partings using the SIEMENS D5000 X-ray diffractometer with CuKα radiation, run from 5 to 60° 2θ, with a step increment of 0.02° and a counting time of 2 s per step. The minerals in each case were identified from the diffractograms by referencing to the ICDD Powder Diffraction File. 4. Results and discussion The analytical results showed that the concentrations of Cd, Co, Se, and U were below the detection limit for all the samples, whereas Ag and Be were detected in less than 50% of the samples (Table 2). As such, these elements were excluded from the statistical analyses, and

1.25

1.14

1.56

1.77

1.75

1.26

0.87 1.99

1.08

not discussed in further detail in this paper. Table 3 shows the range, standard deviation, median, intercept of the element contents in the correlation with the ash yield, the enrichment factor for Mukah coal, the detection limit, the world average for S and minor elements and the Clarke values for trace elements in the world low-rank coals. Data of the S and minor elements for world coal were adopted from Valković (1983), while data of the Clarke values for trace elements in the low-rank coals were adopted from Ketris and Yudovich (2009), and references therein. All analysis results are presented to three significant figures. 4.1. Geochemistry of minor elements Box-plots showing concentrations of the six minor elements in Mukah coal along with their respective world averages are displayed in Fig. 3. The statistical analysis reveals that Al (r=+0.81), K (r=+0.91), and Mg (r=+0.78) are strongly correlated with ash yield (Table 4). The significant positive relationship suggests that these elements are inorganically associated and have contributed significantly to the ash. 4.1.1. Aluminium The Al concentration of Mukah coal is relatively high compared with that of the world average, varying from 1140 to 68,200 ppm (Fig. 3a; Table 3). High Al in coal is related to the presence of clay minerals, and high Al concentration in Mukah coal is due to the presence of non-coal partings. This suggests that clay minerals make up a substantial portion of the constituent minerals. The interrelationship with K and Mg (Table 4) suggests the presence of illite/smectite mixed layer clays as indicated by the SEM–EDX and XRD studies carried out on low-temperature ash (Figs. 4, 5). The stronger relationship between Al with Mg (r= +0.88) compared to K (r=+0.79) indicates higher smectite to illite ratio in the mixed layer clays. The presence of illite/smectite series mixed layer clays is usually related to burial diagenesis and hydrothermal alteration events

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Table 2 Ash yield (%, as-received basis) and concentration of elements (ppm, as-received basis) in the Mukah coals (on whole coal basis). Concentrations of Cd, Co, Se, and U were below the detection limit for all samples; as such they are not listed in the table. Potentially hazardous trace elements are underlined and in bold. Sample

Ag

Al

As

Ba

Be

Ca

Cr

Cu

Fe

K

Mg

Mn

Na

Ni

Pb

S

Sb

Th

V

Zn

Ash

M01/2 M01/1P M01/1 M02/3 M02/2 M02/1 M03/3 M03/2 M03/1 M04/3 M04/2 M04/1 M05/3 M05/2 M05/1 037/3 037/2 037/1 038/2 038/1 039/3 039/2 039/1 043 C 043B 045 046B

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 10 bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl

4880 68,200 35,700 21,300 31,600 5570 37,700 27,900 5620 7240 60,800 1500 4700 31,700 3890 35,800 27,900 3740 41,900 11,500 23,000 27,500 2630 1140 2480 2340 8850

102 107 131 78 78 130 102 88 83 167 104 113 90 65 67 79 112 124 88 122 76 71 73 123 205 84 99

118 173 82 106 108 30 125 129 22 149 217 16 184 113 62 179 148 50 241 56 112 118 4 17 10 8 30

bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl bdl 1 4 bdl bdl

2640 2160 2460 337 405 531 430 249 441 1210 585 725 6500 3280 4660 758 875 1460 580 753 186 146 243 891 550 683 568

2 39 25 15 26 4 30 35 2 6 54 bdl bdl 22 bdl 66 63 35 71 47 55 55 34 31 41 32 48

35 58 61 31 47 34 41 45 18 46 68 26 29 29 17 71 82 66 79 70 60 80 47 56 71 57 57

9440 9600 8740 6050 5620 2470 2700 2790 716 3560 9920 2100 2760 5000 4300 15,400 6510 3310 9860 3140 3500 3600 1080 4950 22,300 1490 2520

480 10,400 5470 4050 8430 526 10,500 17,900 549 973 14,700 173 456 5320 492 14,400 12,100 734 17,700 4840 11,900 14,500 469 307 585 530 2880

463 3780 2240 438 1280 154 1110 1500 111 605 4680 286 1450 2350 1580 3510 2210 466 3420 193 1330 1690 98 399 232 153 596

91 77 58 26 33 14 16 20 6 54 96 38 67 53 98 422 131 78 107 39 30 42 15 79 116 17 47

99 502 4330 94 259 103 248 489 bdl 69 543 73 869 875 1000 379 337 46 507 117 401 352 bdl 69 bdl 47 68

bdl bdl bdl 4 1 bdl bdl 2 bdl bdl 2 bdl bdl bdl 5 23 19 18 20 23 22 16 16 9 23 21 15

213 194 234 124 141 220 152 124 107 283 179 187 137 92 90 181 257 269 197 234 172 185 172 273 362 200 237

1.00 0.82 0.71 0.79 1.86 0.92 0.77 0.69 0.66 0.67 0.77 0.96 0.97 0.82 0.98 0.74 0.88 0.96 0.84 0.96 0.81 0.70 1.02 1.35 2.95 1.69 0.90

80 83 87 74 77 87 83 83 76 83 83 79 76 64 68 62 80 76 62 82 60 66 73 81 99 71 73

9 22 18 5 8 6 10 8 2 15 27 8 8 13 8 25 19 13 23 11 14 18 7 14 23 10 14

5 73 45 31 52 3 55 92 5 4 86 bdl 5 49 4 65 47 2 77 19 62 64 bdl bdl bdl bdl 33

37 56 66 129 63 82 75 54 70 34 118 41 31 33 30 37 18 18 62 108 45 30 15 10 46 54 514

8.13 48.20 33.10 20.70 33.30 3.06 24.60 46.50 2.89 5.35 69.40 0.00 4.53 42.70 5.16 66.10 37.80 7.22 49.20 20.00 58.10 76.20 3.18 2.51 6.38 5.18 23.40

bdl—below detection limit.

(Poppe et al., 2001; Velde, 1992). The illite/smectite series mixed layer clays form when coalification attains the subbituminous coal stage (Rm,oil = 0.4–0.5%), and disappear when an average temperature of 150 °C (Rm,oil = 1%) is attained (Heroux et al., 1979; Stach

et al., 1982; Tucker, 1981). However, the clays can also form when in contact with the K- or Mg-containing solutions in alkaline environments, and therefore it is not a valid indicator of the temperature (Dunoyer de Segonzac, 1970; Heling and Teichmüller, 1974; Stach

Table 3 The range, standard deviation, median, intercept of the element contents in the correlation with ash yield, the enrichment factor for Mukah coal, the detection limit of trace elements in the present study, the world average for S and minor elements and the Clarke values and detection limit for trace elements for low-rank coals (all values in ppm, unless otherwise indicated; as-received basis). Potentially hazardous trace elements are underlined and in bold. Element

Minimum

Maximum

Std. deviation

Median

Intercept

World averagea

Ash (%) Al Ca Fe K Mg Na S (%) Element Ag As Ba Be Cd Co Cr Cu Mn Ni Pb Sb Se Th U V Zn

0.00 1140 146 716 173 98 bdl 0.66 Minimum bdl 65 4 bdl bdl bdl bdl 17 6 bdl 90 60 bdl 2 bdl bdl 10

76.20 68,200 6500 22,300 17,900 4680 4330 2.95 Maximum 10 205 241 4 bdl bdl 71 82 422 23 362 99 bdl 27 bdl 92 514

23.80 18,700 1500 4780 6180 1280 828 0.48 Std. deviation nc 31.6 68.2 nc nc nc 22.0 19.2 78.6 9.51 63.8 9.01 nc 6.64 nc 31.4 93.8

20.70 11,500 683 3600 4050 1110 248 0.88 Median nc 99 108 nc nc nc 32.0 56.0 53 4.0 187 77 nc 13 nc 13.0 46.0

– 3350 – – –142 245 – – Intercept – – 46.8 – – – 18.5 39.6 – – – 80.4 – 8.7 – 3.9 –

– 10,000 10,000 10,000 100 200 200 2 Coal Clarke valuesb 0.090 ± 0.020 7.6 ± 1.3 150 ± 20 1.2 ± 0.1 0.24 ± 0.04 4.2 ± 0.3 15 ± 1 15 ± 1 100 ± 6 9.0 ± 0.9 6.6 ± 0.4 0.84 ± 0.09 1.0 ± 0.15 3.3 2.9 ± 0.3 22 ± 2 18 ± 1

nc—not calculated because detected in less than 50% of the samples. nr—not relevant. a After Valković, 1983. b After Ketris and Yudovich, 2009.

Detection limit – – – – – – – – Enrichment factor nc 13.0 0.7 nc nc nc 2.1 3.7 0.5 0.4 28.3 91.7 nc 3.9 nc 0.6 2.6

nr 1 0.05 0.1 1 0.04 0.5 nr Detection limit 0.6 2 0.03 0.09 0.1 0.2 3 0.4 0.1 0.5 1 2 4 2 10 0.5 0.2

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Fig. 3. Box-plots of concentration for (a) Al, (b) K, (c) Mg, (d) Fe, (e) Ca, (f) Na; the world average is marked with the symbol ‘x’.

et al., 1982). Nevertheless, the presence of the illite/smectite series mixed layer clays in the Mukah coal is consistent with its degree of maturity, and indicates that the mixed layer clays in the Mukah coal might have formed as a result of coalification. 4.1.2. Potassium Concentration of K in the Mukah coal (4050 ppm) is substantially higher than that of the world average (100 ppm) (Fig. 3b; Table 3). Higher K concentration in Mukah coal is mainly due to the enrichment of mixed layer clays in the coal (see Section 4.1.1), as opposed to the enrichment of kaolinite in much of the world's coals (Vassilev and Vassileva, 1996), including coals from the United Kingdom (Spears and Zheng, 1999). 4.1.3. Magnesium Magnesium in coal is related to clay minerals, particularly smectite and chlorite, as well as to carbonates (Spears and Zheng, 1999). In Mukah coal, Mg is mainly associated with the mixed layer clays as discussed above, but not with carbonates. This association is shown by the significant relationships with Al and K, and the lack of good relationship with Ca (Table 4). Magnesium concentration in Mukah coal is 1110 ppm, much higher than the world average of 200 ppm (Fig. 3c; Table 3). The higher Mg concentration is due to the enrichment of mixed layer clays in Mukah coal as opposed to the enrichment of kaolinite in the world coals (see Section 4.1.2). 4.1.4. Iron Iron concentration varies from 716 ppm to 22,300 ppm in Mukah coal, and is higher than the world average (Fig. 3d; Table 3). Extraordinarily high Fe contents were recorded in two samples that showed concentrations of 15,400 ppm and 22,300 ppm. The latter sample also showed higher As (205 ppm) and Pb (362 ppm) concentrations, whereas the former had a higher Mn (422 ppm) concentration (Table 2), suggesting a close relationship between Fe with As, Pb, and Mn.

Iron in coal occurs in a number of forms, such as Fe sulphides (pyrite and marcasite), Fe carbonates (e.g., siderite and ankerite), Fe-bearing clays, Fe sulphates (e.g., ferrous sulphate and jarosite), and organically bound Fe (Wang et al., 2008). As implied by the non-significant positive relationship between Fe and ash yield (r= +0.29), Fe is mostly inorganically associated, though a portion of it might also be organically bound. In Mukah coal, Fe is correlated significantly with S (r= +0.49), but poorly with Ca (r = −0.02) and K (r = +0.23), indicating that the Fe mainly occurs in pyrite. Optical microscopy confirmed that high Fe concentration corresponds with the presence of both syngenetic and epigenetic pyrites (Fig. 6). The presence of syngenetic pyrite is consistent with the brackish depositional environment of the Balingian Formation which favours pyrite precipitation (Casagrande et al., 1977; Dai et al., 2002, 2010a; Diessel, 1992; Renton and Bird, 1991; Stach et al., 1982; Ward, 2002; Widodo et al., 2010).

4.1.5. Calcium Concentration of Ca in Mukah coal (683 ppm) is considerably lower than that of the world average of 10,000 ppm (Fig. 3e; Table 3). The concentration of Ca in Mukah coal varies from 146 to 6500 ppm, with the presence of one outlier and two extreme values (Fig. 3e). All the three samples were markedly high in Ca, representing ply samples collected from the same outcrop. This suggests that peat was deposited in mire supplied by calcium-rich water (Stach et al., 1982). Despite the high Ca concentration, no carbonate minerals were observed in these samples by SEM–EDX and XRD methods (Fig. 5). Carbonate minerals such as calcite and ankerite are rarely found in Mukah coal. Thus, it is reasonable to believe that most of the Ca is organically associated. This is further supported by the non–significant negative relationship with ash yield (r= −0.23), which implies that the Ca is mostly organically associated, though there is also a portion of it probably inorganically bound. This agrees well with the conclusion that a substantial portion of Ca in low-rank coals is carboxyl-bound Ca rather than carbonate (Karayigit et al., 2000; Mukhopadhyay et al., 1998; Spears and Tewalt, 2009; Wang et al., 2008).

1 0.04 1 0.09 0.92⁎⁎

Zn

1 0.52⁎⁎ 0.01 0.63⁎⁎

V

1 0.01 − 0.25 − 0.01 − 0.39⁎

Th

1 0.40⁎ 0.10 − 0.35 − 0.08 − 0.32

Sb

1 0.43⁎ 0.31 − 0.31 0.39⁎

− 0.29 0.07 − 0.20

Fig. 4. SEM photomicrograph of mixed layer clays (I/S) in the low-temperature ash showing a set of well developed cleavages (Sample 046B; secondary electron image).

0.01 0.06 0.19

1 0.46⁎ 0.53⁎⁎ 0.46⁎

S Pb

4.1.6. Sodium Concentration of Na in Mukah coal ranges up to 4330 ppm. Sodium in coal is mainly associated with the organic matter (Ward et al., 1999) or is contained in pore solutions (Spears and Zheng, 1999). The non-significant positive relationship with ash yield (r=+0.20) may imply that Na is mostly inorganically associated, though a portion of these elements is also organically bound. Nevertheless, the nature of the inorganically associated Na is not clear, as Na in the Mukah coal does not correlate with any of the elements analyzed (Table 4).

1 − 0.25 − 0.07 − 0.21 0.09 0.20 0.22 − 0.07 0.20

Ni

1 0.04 0.36 0.16 0.01 − 0.23 0.60⁎⁎ 0.22 − 0.12 0.36

Na

− 0.07 0.78⁎⁎ − 0.03 0.91⁎⁎

Box-plots showing concentration of the eleven trace elements in Mukah coal along with their respective Clarke values are displayed in Fig. 7. When comparing the concentrations of trace elements for the Mukah coal with those of their respective Clarke values for the world low-rank coals reported by Ketris and Yudovich (2009), it is found that the concentrations of Ag, Ba, Be, Cd, Co, Mn, Ni, Se, U, and V in the Mukah coal are lower than their respective Clarke values for the world low-rank coals. Conversely, concentrations of As, Cr, Cu, Pb, Sb, Th, and Zn are higher than those of their respective Clarke values for the world low-rank coals (Table 3). Therefore, considering the PHTEs concentrations in Mukah coal, Be, Cd, Co, Mn, Ni, Se, and U may be of little or no health and environmental concerns, whereas As, Cr, Pb, Sb, and Th require further examination for their potential health and environmental concerns.

0.04 0.55⁎⁎ − 0.13 0.67⁎⁎

0.30 − 0.34 − 0.02 − 0.35

0.38 0.16 − 0.13 − 0.22 − 0.40⁎ − 0.25 0.52⁎⁎ 0.71⁎⁎

0.23 0.30 − 0.15 − 0.23 − 0.31 − 0.09 0.60⁎⁎ 0.89⁎⁎ 0.00 0.81⁎⁎

U—potentially hazardous trace elements. ⁎ Correlation is significant at the 0.05 level (2-tailed). ⁎⁎ Correlation is significant at the 0.01 level (2-tailed).

0.15 0.72⁎⁎

− 0.04 0.72⁎⁎ 0.27 0.02 − 0.27 0.73⁎⁎ 0.63⁎⁎

0.37 0.04 0.71⁎⁎ 0.59⁎⁎ 0.15 − 0.01 0.78⁎⁎ 0.42⁎

1 0.43⁎ 0.54⁎⁎ 0.38⁎

1 0.89⁎⁎ 0.36 0.72⁎⁎ 0.51⁎⁎ 0.41⁎

1 − 0.47⁎ − 0.42⁎ − 0.02 − 0.32 0.15 0.09 0.38 − 0.36 − 0.26 − 0.09 − 0.07 − 0.11 − 0.24 − 0.18 − 0.23 1 − 0.21 − 0.11 − 0.06 0.26 0.44⁎ − 0.31 − 0.24 0.02 0.02 0.06 0.85⁎⁎ 0.47⁎ 0.79⁎⁎

1 0.20 0.36 0.28 0.29 0.72⁎⁎ 0.81⁎⁎

4.2. Geochemistry of trace elements

0.26 0.71⁎⁎ 0.22 − 0.11 0.29

1 0.50⁎⁎ 0.35 − 0.05 − 0.21 − 0.31 − 0.24 0.73⁎⁎ 0.78⁎⁎ 1 0.23 0.42⁎ 0.64⁎⁎ 0.14 0.26 0.43⁎ 0.49⁎⁎

1 0.73⁎⁎ 0.30 0.11 0.18 − 0.21 − 0.32 − 0.29 0.54⁎⁎ 0.95⁎⁎

Mn Mg K Fe Cu Cr Ca Ba As Al

1 − 0.21 0.73⁎⁎ − 0.11 0.51⁎⁎ 0.37 0.32 0.79⁎⁎ 0.88⁎⁎

Al As Ba Ca Cr Cu Fe K Mg Mn Na Ni Pb S Sb Th V Zn Ash

Table 4 Correlation matrix of ash yield and elemental composition for Mukah coal. Potentially hazardous trace elements are underlined and in bold.

185

1

Ash

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4.2.1. Arsenic Arsenic has been identified as one of the PHTEs (US Public Law, 1990), and also as a regulated constituent under Subtitle D of the Resource Conservation and Recovery Act (RCRA) as solid wastes for its potential to contaminate groundwater by leaching when the coal combustion waste is disposed as landfills and surface impoundments (US EPA, 1976). The health and environmental threats posed by As in coal use are (a) the emission problem from coal combustion and (b) its potential toxicity in groundwater if it is leached from coal-mining waste or from fly ash at disposal sites (Kolker et al., 2005; Miller, 2011; Schweinfurth, 2003; Swaine and Goodarzi, 1995; Zheng et al., 1999). Endemic arsenosis in the Guizhou Province, China has resulted in the deaths of 265 people from 1976 to 2003 (Yang, et al., 2006). More than 10,000 people were affected while 3000 suffered from severe arsenosis (Dai et al., 2011a; Ding et al., 2001; Zheng et al., 1996). The endemic disease was generally believed to have been caused by domestic combustion of As-rich coal for cooking, home heating, food drying

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Fig. 5. X-ray diffractogram showing the presence of mixed layer clays and quartz in the low temperature ash for samples; (a) 046B, (b) M04-2, (c) M05-2.

in unventilated residential environments (Dai et al., 2005; Ding et al., 2001; Finkelman et al., 1999; Fuge, 2005; Wang et al., 2006; Zhao et al., 2008; Zheng et al., 1996), as well as by the high-As clay binder used for coal briquette (Dai et al., 2011a). Arsenic poisoning has been cited as a cause of marked hearing loss for children and higher rates of non-melanoma skin cancer for those who

lived within the zone of influence of a coal-fired power plant in the Czech Republic. The poisoning has been traced to the As emitted from the coal-fired power plant burning of high-As lignite (900–1500 ppm) (Bencko and Symon, 1977; Finkelman, 2003; Thornton et al., 2003). The concentration of As varies from 65 to 205 ppm (Fig. 7a; Table 3). With an enrichment factor of 13.0 relative to the Clarke value for the

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1976). So far, there have been no reports of environmental or health impacts from Ba present in coal (Swaine, 1990). The enrichment factor for Ba in Mukah coal is 0.72 relative to the Clarke value for the world lowrank coals, suggesting that Ba is depleted in Mukah coal as compared with the Clarke concentration of Ba for low-rank coals (Fig. 7b; Table 3). Therefore, its concentration, as well as its toxicity, in Mukah coal is too low to be of any health and environmental concerns. In coals, Ba has been recorded in barite (BaSO4) (Swaine, 1990), in carbonate as witherite (BaCO3) (Wang et al., 2008) or as a solidsolution of aluminophosphates in the Ba-bearing aluminophosphate (gorceixite) or crandallite [CaAl3(PO4)2(OH)5·(H2O)] (Dai et al., 2011b; Wang et al., 2008; Ward et al., 1996). The strong positive relationship of Ba with ash yield (r = + 0.67) and the small interception at axis Ba on the plot of Ba against ash yield (Table 3) suggests that Ba is predominantly inorganically associated, and only a small portion of Ba occurs in the organic matter. The significant negative relationship of Ba with S (r = − 0.40) rules out the association of Ba as barite. In contrast to the documented association of Ba, the strong relationships with Al (r = + 0.73), K (r = + 0.72), and Mg (r = + 0.81) suggest that mixed layer clays may be an important host to Ba in Mukah coal (Table 4).

Fig. 6. Photomicrographs showing the presence of (a) syngenetic pyrite, (b) epigenetic infilling pyrite. Photomicrographs taken on polished surfaces under reflected light and oil immersion, (Sample 043B).

world low-rank coals reported by Ketris and Yudovich (2009), Mukah coal is highly enriched in As. Therefore, As in the Mukah coal is of serious health and environmental concerns. As a preventive measure, mining for coals exceeding 100 ppm As has been forbidden in the endemic-arsenosis areas in China (Dai et al., 2011a; Yudovich, and Ketris, 2005), but according to Russian official norms, the “coal threshold of toxicity” has been set at 300 ppm (Yudovich, and Ketris, 2005; Zharov et al., 1996). Previous studies have shown that a substantial fraction of As is either organically associated or present in form of arsenate in the low-rank coals (Belkin et al., 1997; Finkelman and Greb, 2008; Kolker et al., 2005; Swaine, 1990; Yudovich, and Ketris, 2005). This is also true for Mukah coal as indicated by the negative relationship with ash yield (r= −0.35) that is statistically non-significant. Arsenic in Mukah coal is also inorganically associated within sulphide minerals, particularly pyrite. This is indicated by the significant positive relationship with Fe (r= +0.44) and S (r =+0.47) (Table 4). The finding is consistent with the documented association of As with pyrite and other sulphides (Belkin et al., 1997; Dai et al., 2006b; Finkelman, 1994; Hower et al., 1997; Ward et al., 1996; Yudovich, and Ketris, 2005). Although the association of As with clays has also been documented (Belkin et al., 1997; Finkelman, 1994; Yudovich, and Ketris, 2005), no significant relationship between As and the clay-forming elements has been established in this study to suggest this association. 4.2.2. Barium Although Ba is not included as a PHTE by the United States Environmental Protection Agency (US Public Law, 1990), it has been listed as a regulated constituent under Subtitle D of RCRA as solid wastes (US EPA,

4.2.3. Chromium Although Cr has been identified as one of the PHTEs (US Public Law, 1990), and also as a regulated constituent under Subtitle D of RCRA as solid wastes (US EPA, 1976), there has so far been little evidence of deleterious effects from Cr in coal mining and utilisation (Swaine, 1990). However, Ren et al. (2004) showed that the lung cancer in the Shengbei Coalfield of China was associated with high Cr and other toxic trace elements in lignite. Mukah coal is 2.1 times enriched in Cr compared to the Clarke value for the world low-rank coals (Fig. 7c; Table 3). Although the modes of occurrence of Cr in coals are not very clear (Swaine, 1990), Cr seems to be associated with organic matter (Dai et al., 2008b; Finkelman, 1981; 1994; Mukherjee et al., 1988; Ren et al., 2004), clay minerals (Rimmer, 1991), and may also be present as very small chromite (FeCr2O4) grains (Finkelman, 1981; 1994). The inorganic association of Cr in the Mukah coal is demonstrated by the positive relationship with ash yield (r=+0.72). Nevertheless, a fraction of the Cr is also organically associated as evidenced by the interception at axis Cr on the plot of Cr against ash yield (Table 3). Consistent with the documented association of Cr with clays, the significant relationships with Al (r=+ 0.51), K (r=+0.72), and Mg (r=+0.51) as revealed in the statistical analysis, indicates the association of Cr with mixed layer clays. The absence of carbonate-bound Cr in Mukah coal can be deduced from the significant negative relationship between Cr and Ca (r=−0.47). 4.2.4. Copper Copper is neither a PHTE (US Public Law, 1990) nor a regulated constituent under Subtitle D of RCRA as solid wastes (US EPA, 1976). Concentration of Cu in Mukah coal ranges from 17 to 82 ppm (Fig. 7d; Table 3). Although the concentration of Cu is 3.7 times higher than its Clarke value, it is not of health and environmental concerns due to the low toxicity. The statistical analysis reveals that though Cu is mostly inorganically associated in Mukah coal, a substantial portion of it is also organically associated. This is suggested by the significant relationship with ash yield (r= +0.55) and the large interception at axis Cu on the plot of Cu against ash yield (Table 3). The association of Cu with chalcopyrite is demonstrated by the significant relationship with Fe (r = +0.43). Nevertheless, no significant relationship with sulphur was found in this study. This is probably due to the mainly organic association of sulphur in Mukah coal (Table 5) which is also suggested by the negative relationship between S and ash yield (r= −0.32). Copper in Mukah coal is also bound within the clay minerals as demonstrated by the significant relationship with K (r= +0.54) and Mg (r= +0.38) (Table 4). The significant negative relationship between Cu and Ca (r = −0.42), as revealed in this study, indicates the non-association of Cu with the

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Fig. 7. Box-plots of concentration for (a) As, (b) Ba, (c) Cr, (d) Cu, (e) Mn, (f) Ni, (g) Pb, (h) Sb, (i) Th, (j) V, (k) Zn; Clarke values are marked with symbol ‘x’.

carbonates. The association of Cu in Mukah coal with the sulphide minerals, particularly the chalcopyrite (CuFeS2), the clay minerals and the organic compounds, is in agreement with the results of other researchers (Finkelman, 1981; 1995; Finkelman and Bunnell, 2003; Finkelman and Gross, 1999; Swaine, 1990).

4.2.5. Manganese Although Mn is one of the PHTEs (US Public Law, 1990), there is no reported health or environmental problems from Mn in coal (Swaine, 1990). Manganese is depleted in Mukah coal as indicated by the enrichment factor (0.53). The range of Mn concentrations is broad,

S.G. Sia, W.H. Abdullah / International Journal of Coal Geology 88 (2011) 179–193 Table 5 Total S, sulphate S, pyritic S and organic S contents of selected Mukah coal samples (all values in wt.% and on whole coal and as-received basis).

Total S Sulphate S Pyritic S Organic S

M02/1

M05/3

045

0.92 0.08 0.08 0.75

0.97 0.00 0.19 0.78

1.69 0.00 0.25 1.44

189

Although the modes of occurrence of Ni in coal are still not precisely known (Finkelman, 1994), previous work suggests the association of Ni with clays (Finkelman, 1988), carbonates (Finkelman, 1994), sulphides (Swaine, 1990), as well as organically bound Ni (Dai et al., 2008b; Finkelman, 1981; 1994). Nickel in Mukah coal correlates poorly with ash yield (r= +0.19); this implies that Ni is mostly inorganically associated. Nevertheless, in the present study, no significant relationship was found with all the three key elements, namely K (r= +0.18), Fe (r= +0.26), and Ca (r= −0.36), to deduce its inorganic association.

varying from 6 to 442 ppm (Fig. 7e; Table 3). The broad range usually indicates an association with discrete minerals. The presence of organically bound Mn is common in the low-rank coals (Finkelman, 1993; Swaine, 1990). In Mukah coal, Mn is mostly inorganically associated, though a portion of it is also organically associated. This is implied by the positive relationship with ash yield (r= +0.36). Manganese in coal is predominantly associated with carbonate minerals, particularly with iron carbonate (Finkelman, 1994; Huggins and Huffman, 1996; Swaine, 1990). Manganese and Mg are known to substitute for Fe in siderite [FeCO3] forming a complete solid solution between siderite and rhodochrosite [MnCO3] and between siderite and magnesite [MgCO3] (Deer et al., 1985). As such, it is reasonable to believe that the significant relationship with Fe (r= +0.64) and Mg (r= +0.50) in Mukah coal is due to its association with the carbonates. The close relationship between Mn and Fe is also displayed by the proximity between the two elements in the dendogram (Fig. 8). Although Mn is also reported to occur within clay minerals (Finkelman, 1994; Garcia et al., 1994; Huggins and Huffman, 1996; Swaine, 1990), no significant relationship with the clay-forming elements has been observed in the present study to support such an association.

4.2.7. Lead Lead is both a PHTE and a regulated constituent under Subtitle D of RCRA as solid wastes. Nevertheless, so far there has not been any report of adverse health or environmental impacts from Pb in coal (Swaine, 1990). The dispersion range of Pb is broad and varies from 90 to 362 ppm (Fig. 7g; Table 3). Mukah coal is 28.3 times enriched in Pb as compared with the Clarke value for Pb in the world low-rank coals. Lead occurs mainly as sulphide minerals, with galena (PbS) being the most common form (Dai et al., 2006a; Finkelman, 1994; Swaine, 1990). Occurrences of Pb selenide (PbSe) (Finkelman, 1994) and crocoite (PbCrO4) (Li et al., 2001) have also been reported in coal. An organic association of Pb may also be possible, especially in the low-rank coals (Swaine, 1990). The non-significant negative relationship with ash yield (r= −0.20) implies that though Pb is both organically and inorganically associated in the Mukah coal, the organically bound Pb is by far more dominant. Lead in the Mukah coal is also associated with galena and pyrite, as shown by its significant relationship with S (r=+0.46) and Fe (r=+0.43). Lead may not occur as Pb selenide in Mukah coal as evidenced by the very low Se concentration.

4.2.6. Nickel Despite being a PHTE, there is no reported health or environmental problems from Ni in coal mining and utilisation (Swaine, 1990). Though localised enrichment of Ni, ranging from 33 to 304 ppm on the wholecoal basis, has been reported in lignite from the Kosovo Basin, southern Serbia (Ruppert et al., 1996), it is depleted in Mukah coal. The enrichment factor for Ni is 0.44, suggesting that concentration of Ni is about half the Ni Clarke value in low-rank coals (Fig. 7f; Table 3).

4.2.8. Antimony Despite being a PHTE (US Public Law, 1990), there is no report of adverse environmental impacts from the Sb compounds present in coal and coal ash (Swaine, 1990). Anomalous Sb content in coal is known in the Almalik deposit, Uzbekistan, and in the Late Permian anthracite from Xingren, Guizhou, China. In the Almalik deposit the highest Sb concentration in coal was reported to be 5800 ppm, though the mean concentration for this coal is 56 ppm (Eskenazy,

Dendrogram using Average Linkage (Between Groups) Rescaled Distance Cluster Combine CASE 0 5 10 15 Label

20

25

K V Ash yield Al Mg Ba Fe Mn Cr Cu Th Ni Zn Ca Na As Pb Sb S Fig. 8. Dendogram using average linkage (between groups); three distinct clusters can be recognised (potentially hazardous trace elements are underlined and in bold).

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1995). In Guizhou, China anomalous Sb concentration as high as 0.05%, with an average of 0.02%, has also been reported (Dai et al., 2006b), though the arithmetic mean of Sb concentration in the Chinese coals is estimated to be only 0.84 ppm (Dai et al., 2011a). Antimony in this area occurs as fracture-fillings kaolinite of hydrothermal origin, in an epigenetic getchellite [AsSbS3] (Dai et al., 2006b). Mukah coal is highly enriched in Sb, with concentration ranging from 60 to 99 ppm (Fig. 7h; Table 3). The enrichment factor of Sb for Mukah coal is 91.7 (Table 3) indicating that concentration of Sb is 91.7 times the Clarke value for Sb in the world low-rank coals. Antimony is both organic and inorganic associated in coal. The organically association of Sb in Mukah coal is shown by the significant negative relationship with ash yield (r =−0.39) (Table 4). Antimony may also be present in solid solution in pyrite, as minute accessory sulphides, probably as stibnite (Sb2S3), dispersed within the organic matter (Finkelman, 1994; Karayigit et al., 2000; Qi et al., 2008), or as getchellite (AsSbS3) (Dai et al., 2006b). While the statistical analysis performed shows a non-significant statistical relationship with Fe (r= +0.26), Sb is significantly correlated with As (r=+0.79) and S (r=+0.40). It is reasonable to believe that Sb in Mukah coal is not concentrated in pyrite, but is mainly associated with getchellite (AsSbS3), and probably also with stibnite (Sb2S3). 4.2.9. Thorium Thorium has been identified as one of the PHTEs (US Public Law, 1990). Environmental concerns of Th in coal are primarily due to its radioactivity (Finkelman, 1994); although there is no evidence to suggest that radioactivity from coal or coal ash has caused any human health problems (Finkelman, 2000). Mukah coal has a Th enrichment factor of 3.9 relative to the Clarke value (Fig. 7i; Table 3). It has been shown in the literature that high concentration of Th in coal is always related to the presence of the mineral monazite, and to a lesser extent of zircon and possibly xenotime. In this study, however, these minerals were not detected. The absence of these minerals is also supported by the narrow dispersion range of Th concentration (2 to 27 ppm), as these minerals usually contain higher concentrations of Th. The statistical significant relationship with ash yield (r = +0.63) indicates the inorganic association of Th. Nevertheless, organically associated Th might be also present as indicated by the interception at the Th axis of Th against the ash yield plot (Table 3). Small amounts of Th could also be present in iron oxides and clays (Finkelman, 1995; Finkelman and Bunnell, 2003; Finkelman and Gross, 1999; Swaine, 1990). The strong relationship (r=+0.71) with Fe, and with the aluminosilicate elements (Table 4), such as Al (r =+0.60), K (r= +0.54), and Mg (r= +0.73) are consistent with the documented modes of occurrence of Th. 4.2.10. Vanadium Vanadium is neither a PHTE (US Public Law, 1990) nor a regulated constituent under Subtitle D of RCRA as solid wastes (US EPA, 1976), though it can be leached from coal ash and poses adverse health or environmental impacts. Nevertheless, so far there has not any report of adverse health or environmental impacts from V contained in coal and coal ash. Anomalous V content in coal has been found in the Western Kentucky No. 9 coal bed (Hower et al., 2000), as well as in the Heshan coalfield, Guangxi province (Zeng et al., 2005); Zhijin deposit, Guizhou province (Dai et al., 2004); Yanshan Coal field, Yunnan Province (Dai et al., 2008b) in China, with concentrations of 1.06%, 0.14%, 0.06%, and 0.057% on a whole coal basis, respectively. However, Mukah coal was found to be depleted in V, as suggested by the enrichment factor of 0.59. Good relationships with Al (r = + 0.89), K (r = + 0.95) and Mg (r = +0.78), as revealed in the statistical analyses, are consistent with the well known association of V with the mixed layer clays. Although a portion of V is also reported to be organically associated (Dai et al., 2008b; Finkelman, 1995; Finkelman and Bunnell, 2003; Finkelman and Gross, 1999), the close relationship with ash yield (r = +0.92), along with the close to zero interception at the plot of

V against ash yield (Table 3), suggests that V in the Mukah coal is exclusively inorganically bound. 4.2.11. Zinc Zinc is not identified as a PHTE and is also not a regulated constituent under Subtitle D of RCRA as solid wastes (US EPA, 1976). So far, there are no deleterious health or environmental impacts from Zn emissions from the utilisation of coal in coal-fired power stations or by leaching of Zn from the disposed coal ash (Swaine, 1990). The enrichment factor for Zn is 2.6, suggesting that Zn is enriched in Mukah coal (Table 3). Despite this, it is not of any health and environmental concerns due to the low toxicity. In Mukah coal, Zn concentration has a wide variation ranging from 10 to 514 ppm, with the presence of an outlier and an extreme value noted in this study (Fig. 7k). The presence of Zn as sphalerite (ZnS) is detected in Mukah coal (Fig. 9), which is also consistent with previous observations (Finkelman, 1982; Goodarzi, 2002; Mastalerz and Drobniak, 2007; Swaine, 1990; Wang et al., 2008). The lack of apparent relationship between Zn and ash yield (r = + 0.04) implies that Zn is probably contained in both the organic and inorganic matters (Table 4). Though Zn is reported to be mainly associated with pyrite (Finkelman, 1981; Goodarzi, 2002; Wang et al., 2008), this study showed no relationship between zinc and iron (r=−0.11). Neither was there correlation with other index elements, such as K (r=−0.03) and Ca (r=−0.18). This convinces us that the inorganic portion of Zn in Mukah coal is mainly bound within the sphalerite. 4.3. Cluster analysis Cluster analysis was carried out to identify the similarity of elements in the coal by grouping a set of data into clusters for better visualisation. In the present analysis, three clusters can be identified in the dendrogram: one cluster including the elements predominantly associated with mixed layer clays, another cluster that includes the elements mostly inorganically associated, and the third cluster of mainly organically associated elements (Fig. 8). The predominantly inorganically associated cluster is made up of K, V, Al, Mg, and Ba, and these are closely related to ash yield. The close relationship with Al, K and Mg reflects the influence of clay minerals, particularly the mixed layer clays. These elements contribute substantially to the formation of ash. Vanadium and Ba are the two trace elements in this cluster which are predominantly associated with mixed layer clays; they can be removed by physical cleaning technologies. Upon combustion, these elements tend to be retained in the bottom ash and the fly ash particles. Iron, Mn, Cr, Cu, Ni, and Th are mostly inorganically bound in varying proportions within the aluminosilicate, sulphide and/or carbonate

Fig. 9. BSE photomicrograph of sphalerite (S) embedded in coal (C) (Sample M04-2).

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minerals, though a portion of these elements is also organically associated. Only the portion of inorganically associated trace elements in this cluster can be removed by physical cleaning technologies; they are otherwise retained in the ash upon combustion. The dendrogram also reveals that Fe is closely related to Mn, whereas Cr, Cu, Th, and Ni are closely related to one another. Arsenic, Pb, Sb, and S are mostly organically associated elements, though a substantial portion of these elements are also associated with the sulphide minerals. As these elements are mostly organically associated in Mukah coal, they are only sparingly removed by physical cleaning technologies (such as coal washing). They are, therefore, likely to be vaporised during coal combustion, with some of it released into the atmosphere and some adsorbed on the fine fly ash particles. Sodium, Ca and Zn display single isolated long stems that suggest an absence of strong association of the level of these elements in Mukah coal with the ash yield and all the other elements. The elements appear to be both organically and inorganically associated. 5. Conclusion Mineral matter in the Mukah coal is mainly made up of clay minerals. Their presence, particularly in mixed layer clays, contributes significantly to the ash. The elements related to aluminosilicates such as Al, K, and Mg, are the main ash-forming elements in Mukah coal. The elements Na, Fe, and Ca are both organically and inorganically associated in the coal, although Na and Fe are relatively more abundant in the inorganic constituents of the coal whereas Ca originates more commonly from the organic constituents. Compared with their respective Clarke values, Mukah coal is depleted in Ag, Ba, Be, Cd, Co, Mn, Ni, Se, U, and V. The coal is, however, enriched in As, Cr, Cu, Pb, Sb, Th, and Zn. Among the trace elements studied, V and Ba are two that are associated predominantly with clay minerals. On the other hand, Mn, Cr, Cu, Th, and Ni are mostly bound within discrete minerals such as aluminosilicates, sulphides and/or carbonates, though a portion of these elements are also organically associated. Arsenic, Pb, and Sb are mostly organically bound, though a portion of these elements are also associated with sulphide minerals. Zinc has been found to be both inorganically and organically associated. From the aspect of health and the environment in the use of Mukah coal, it is important that PHTEs are reduced to safe levels through appropriate coal cleaning or emission control measures. In this regard, it is encouraging that among the PHTEs, the contents of Be, Cd, Co, Mn, Ni, Se and U are present in Mukah coal below their Clarke values. On the other hand, particular attention should be directed to As, Cr, Pb, Sb and Th that are present in concentrations above their respective Clarke levels. Of the latter group, Cr and Th are mostly inorganically associated and can be removed by physical cleaning technologies. They have generally no or very low volatility and tend to be retained in the bottom ash and the fly ash particles upon combustion. Arsenic, Pb and Sb, being mostly organically bound, cannot be removed by physical cleaning technologies. They tend to vaporise during coal combustion and are released as vapours or gases into the atmosphere, or they are adsorbed onto the fine fly ash particles. Acknowledgements The authors would like to sincerely thank Mr. Dorani J. and Mr. Bakar J. of Sarawak Coal Resources Sdn. Bhd., who provided the opportunity to access the mine sites. The authors also wish to thank Dato' Haji Yunus Abd Razak, Director General of the Minerals and Geoscience Department Malaysia, for the use of the scanning electron microscope facilities and for the academic support rendered. The authors also would like to express their gratitude to Mr. Ali Ismail of the Mineral Research Centre of the Minerals and Geoscience Department Malaysia for assistance with the scanning electron microscope. Thanks also go to Mr. Wong, V.C. of the Minerals and Geoscience Department

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Malaysia (Sabah) for field assistance and Ms. Jacinta John for arranging field transportation. We also would like to thank Dr. Ralph L. Kugler for reviewing the first draft of this paper, as well as Dr. Dai, S., Mr. Jaithish John, and the two other anonymous reviewers for their valuable comments. The study received financial support from the University of Malaya IPPP research grant No PS438-2010A.

References Arbuzov, S.I., Volostnov, A.V., Rikhvanov, L.P., Mezhibor, A.M., Ilenok, S.S., 2011. Geochemistry of radioactive elements (U, Th) in coal and peat of northern Asia (Siberia, Russian Far East, Kazakhstan, and Mongolia). International Journal of Coal Geology 86, 318–328. ASTM D5142, 2009. Standard Test Methods for Proximate Analysis of the Analysis Sample of Coal and Coke by Instrumental Procedures. ASTM International, West Conshohocken, PAwww.astm.org. doi:10.1520/D5142-09. 2009. Belkin, H.E., Zheng, B., Zhou, D., Finkelman, R.B., 1997. Preliminary results on the geochemistry and mineralogy of arsenic in mineralized coals from endemic arsenosis areas in Guizhou Province, P.R. China. Fourteenth Annual International Pittsburgh Coal Conference, CD-ROM, (University of Pittsburgh). Bencko, V., Symon, K., 1977. Health aspects of burning coal with a high arsenic content: 1. Arsenic in hair, urine, and blood in children residing in a polluted area. Environmental Research 13, 378–385. Benson, S.A., Holm, P.L., 1985. Composition of inorganic constituents in three low-rank coals. Industrial and Engineering Chemistry Product Research and Development 24, 145–149. Brownfield, M.E., Affolter, R.H., Cathcart, J.D., Johnson, S.Y., Brownfield, I.K., Rice, C.A., 2005. Geologic setting and characterization of coals and the modes of occurrence of selected elements from the Franklin coal zone, Puget Group, John Henry No. 1 mine, King County, Washington, USA. International Journal of Coal Geology 63, 247–275. Casagrande, D.J., Seiffert, K., Berschinski, C., Sutton, N., 1977. Sulfur in peat forming systems of the Okefenokee Swamp and Florida Everglades: origin of sulfur in coal. Geochimica et Cosmochimica Acta 41, 161–167. Clarke, L.B., Sloss, L.L., 1992. Trace Elements—Emissions from the Coal Combustion and Gasification. IEA publication, London, England. (111 pp.). Cohen, A.D., Spackman, W., Dolson, P., 1984. Occurrence and distribution of sulfur in peat-forming environments of southern Florida. International Journal of Coal Geology 4, 74–96. Dai, S., Ren, D., Tang, Y., Shao, L., Li, S., 2002. Distribution, isotopic variation and origin of sulfur in coals in the Wuda coalfield, Inner Mongolia, China. International Journal of Coal Geology 51, 237–250. Dai, S., Li, D., Ren, D., Tang, Y., Shao, L., Song, H., 2004. Geochemistry of the late Permian No 30 coal seam, Zhijin coalfield of southwest China: influence of a siliceous low temperature hydrothermal fluid. Applied Geochemistry 19, 1315–1330. Dai, S., Ren, D., Tang, Y., Yue, M., Hao, L., 2005. Concentration and distribution of elements in Late Permian coals from western Guizhou Province, China. International Journal of Coal Geology 61, 119–137. Dai, S., Ren, D., Chou, C.-L., Li, S., Jiang, Y., 2006a. Mineralogy and geochemistry of the No. 6 coal (Pennsylvanian) in the Jungar Coalfield, Ordos Basin, China. International Journal of Coal Geology 66, 253–270. Dai, S., Zeng, R., Sun, Y., 2006b. Enrichment of arsenic, antimony, mercury, and thallium in a Late Permian anthracite from Xingren, Guizhou, Southwest China. International Journal of Coal Geology 66, 217–226. Dai, S., Zhou, Y., Ren, D., Wang, X., Li, D., Zhao, L., 2007. Geochemistry and mineralogy of the Late Permian coals from the Songzao Coalfield, Chongqing, southwestern China. Science in China Series D: Earth Science 50, 678–688. Dai, S., Li, D., Chou, C.-L., Zhao, L., Zhang, Y., Ren, D., Ma, Y., Sun, Y., 2008a. Mineralogy and geochemistry of boehmite-rich coals: new insights from the Haerwusu Surface Mine, Jungar Coalfield, Inner Mongolia, China. International Journal of Coal Geology 74, 185–202. Dai, S., Ren, D., Zhou, Y., Chou, C.-L., Wang, X., Zhao, L., Zhu, X., 2008b. Mineralogy and geochemistry of a superhigh-organic-sulfur coal, Yanshan Coalfield, Yunnan, China: evidence for a volcanic ash component and influence by submarine exhalation. Chemical Geology 255, 182–194. Dai, S., Wang, X., Chen, W., Li, D., Chou, C.-L., Zhou, Y., Zhu, C., Li, H., Zhu, X., Xing, Y., Zhang, W., Zou, J., 2010a. A high-pyrite semianthracite of Late Permian age in the Songzao Coalfield, southwestern China: mineralogical and geochemical relations with underlying mafic tuffs. International Journal of Coal Geology 83, 430–445. Dai, S., Zhao, L., Peng, S., Chou, C.-L., Wang, X., Zhang, Y., Li, D., Sun, Y., 2010b. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia, China. International Journal of Coal Geology 81, 320–332. Dai, S., Ren, D., Chou, C.-L., Finkelman, R.B., Seredin, V.V., Zhou, Y., 2011a. Geochemistry of trace elements in Chinese coals: a review of abundances, genetic types, impacts on human health, and industrial utilization. International Journal of Coal Geology. doi:10.1016/j.coal.2011.02.003. Dai, S., Zou, J., Jiang, Y., Ward, C.R., Wang, X., Li, T., Xue, W., Liu, S., Tian, H., Sun, X., Zhou, D., 2011b. Mineralogical and geochemical compositions of the Pennsylvanian coal in the Adaohai Mine, Daqingshan Coalfield, Inner Mongolia, China: modes of occurrence and origin of diaspore, gorceixite, and ammonian illite. International Journal of Coal Geology. doi:10.1016/j.coal.2011.06.010. Davidson, R.M., Clarke, L.B., 1996. Trace Elements in Coal. IEAPER/21. IEA Coal Research, London, UK.

192

S.G. Sia, W.H. Abdullah / International Journal of Coal Geology 88 (2011) 179–193

Deer, W.A., Howie, R.A., Zussman, J., 1985. An Introduction to the Rock-Forming Minerals, ELBS. (528 pp.). Diessel, C.F.K., 1992. Coal-bearing Depositional Systems. Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong. (721 pp.). Ding, Z., Zheng, B., Long, J., Belkin, H.E., Finkelman, R.B., Chen, C., Zhou, D., Zhou, Y., 2001. Geological and geochemical characteristics of high arsenic coals from endemic arsenosis areas in southwestern Guizhou Province, China. Applied Geochemistry 16, 1353–1360. Du, G., Zhuang, X., Querol, X., Izquierdo, M., Alastuey, A., Moreno, T., Font, O., 2009. Ge distribution in the Wulantuga high-germanium coal deposit in the Shengli coalfield, Inner Mongolia, northeastern China. International Journal of Coal Geology 78, 16–26. Dunoyer de Segonzac, G., 1970. The transformation of clay minerals during diagenesis and low grade metamorphism, a review. Sedimentology 15, 281–346. Eskenazy, G.M., 1995. Geochemistry of arsenic and antimony in Bulgarian coals. Chemical Geology 119, 239–254. Eskenazy, G.M., Stefanova, Y.S., 2007. Trace elements in the Goze Delchev coal deposit, Bulgaria. International Journal of Coal Geology 72, 257–267. Filippidis, A., Georgakopoulos, A., Kassoli-Fournarakí, A., Misaelides, P., Yiakkoupiś, P., Broussoulis, J., 1996. Trace element contents in cornposited samples of three lignite seams from the central part of the Drama lignite deposit, Macedonia, Greece. International Journal of Coal Geology 29, 219–234. Finkelman, R.B., 1981. Modes of occurrence of trace elements in coal. U.S. Geological Survey Open-File Report, No. OFR-81-99 (301 pp.). Finkelman, R.B., 1982. Modes of occurrence of trace elements and minerals in coal: an analytical approach. In: Filby, R.H., Carpenter, B.S., Ragaini, R.C. (Eds.), Atomic and Nuclear Methods in Fossil Energy Research. Plenum Press, New York, pp. 141–149. Finkelman, R.B., 1988. The inorganic geochemistry of coal: a scanning electron microscopy view. Scanning Microscopy 2, 97–105. Finkelman, R.B., 1993. Trace and minor elements in coal. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 593–607. Finkelman, R.B., 1994. Modes of occurrence of potentially hazardous elements in coal: levels of confidence. Fuel Processing Technology 39, 21–34. Finkelman, R.B., 1995. Modes of occurrence of environmentally sensitive trace elements in coal. In: Swaine, D.W., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht, pp. 24–50. Finkelman, R.B., 2000. Health impacts of coal combustion. USGS Fact Sheet FS-094-00 (2 pp.). Finkelman, R.B., 2003. Coal quality and public health. In: Schweinfurth, S.P. (Ed.), Coal— A Complex Natural Resource, An Overview of Factors Affecting Coal Quality and Use in the United States: U.S. Geological Survey Circular, 1143, pp. 33–35. Finkelman, R.B., Bunnell, J.E., 2003. Short Course A, Health Impacts of Coal: Should We Be Concerned? The Society for Organic Petrology Arlington. (57 slide). Finkelman, R.B., Greb, S.F., 2008. Chapter 10 environmental and health impacts. In: Suárez-Ruiz, I., Crelling, J.C. (Eds.), Applied Coal Petrology—The Role of Petrology in Coal Utilization, pp. 263–287. Finkelman, R.B., Gross, P.M.K., 1999. The types of data needed for assessing the environmental and human health impacts of coal. International Journal of Coal Geology 40, 91–101. Finkelman, R.B., Belkin, H.E., Zheng, B.S., 1999. Health impacts of domestic coal use in China. Proceedings of the National Academy of Sciences of the United States of America 96, 3427–3431. Fuge, R., 2005. Chapter 3 anthropogenic sources. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U., Smedley, P. (Eds.), Essentials of Medical Geology, Impacts of the Natural Environment on Public Health, pp. 43–60. Garcia, A.B., Vega-Jose, M.G., Martinez-Tarazona, M.R., Spears, D.A., 1994. Removal of trace elements from Spanish high rank coals by a selective agglomeration process. Fuel 73, 1189–1196. Given, P.H., Spackman, W., 1978. Reporting of analyses of low rank coals on the dry, mineral-matter-free basis. Fuel 57, 319. Gluskoter, H.J., Ruch, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B., Kuhn, J.K., 1977. Trace elements in coal, occurrence and distribution. Illinois State geological Survey Circular, 499 (154 pp.). Golab, A.N., Carr, P.F., 2004. Changes in geochemistry and mineralogy of thermally altered coal, Upper Hunter Valley, Australia. International Journal of Coal Geology 57, 197–210. Goodarzi, F., 1995. Geology of trace elements in coal. In: Swaine, D.J., Goodarzi, F. (Eds.), Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Amsterdam, pp. 51–75. Goodarzi, F., 2002. Mineralogy, elemental composition and modes of occurrence of elements in Canadian feed-coals. Fuel 81, 1199–1213. Goodarzi, F., Swaine, D.J., 1994. The influence of geological factors on the concentration of boron in Australian and Canadian coals. Chemical Geology 118, 301–318. Gürdal, G., 2011. Abundances and modes of occurrence of trace elements in the Çan coals (Miocene), Çanakkale-Turkey. International Journal of Coal Geology 87, 157–173. Heling, D., Teichmüller, M., 1974. Die Grenze Montmorillonite/Mixed-Layer-Minerale und ihre Beziehung zur Inkohlung in der Grauen Schichtenfolge des Oligozäns im Oberrheingraben. Fortschritte in der Geologie von Rheinland und Westfalen 24, 113–128 (in German). Heroux, Y., Changnon, A., Bertrand, R., 1979. Compilation and correlation of major thermal maturation indicators. American Association of Petroleum Geologists Bulletin 63, 2128–2144. Hower, J.C., Robertson, J.D., Wong, A.S., Eble, C.F., Ruppert, L.F., 1997. Arsenic and lead concentrations in the Pond Creek and Fire Clay coal beds, eastern Kentucky coal field. Applied Geochemistry 12, 281–289.

Hower, J.C., Greb, S.F., Cobb, J.C., Williams, D.A., 2000. Discussion on origin of vanadium in coals: parts of the Western Kentucky (USA) No. 9 coal rich in vanadium. Journal of the Geological Society 157, 1257–1259. Hu, J., Zheng, B., Finkelman, R.B., Wang, B., Wang, M., Li, S., Wu, D., 2006. Concentration and distribution of sixty-one elements in coals from DPR Korea. Fuel 85, 679–688. Huang, Y., Jin, B., Zhong, Z., Xiao, R., Tang, Z., Ren, H., 2004. Trace elements (Mn, Cr, Pb, Se, Zn, Cd and Hg) in emissions from a pulverized coal boiler. Fuel Processing Technology 86, 23–32. Huggins, F.E., 2002. Overview of analytical methods for inorganic constituents in coal. International Journal of Coal Geology 50, 169–214. Huggins, F.E., Huffman, G.P., 1996. Modes of occurrence of trace elements in coal from XAFS spectroscopy. International Journal of Coal Geology 32, 31–53. Karayigit, A.I., Spears, D.A., Booth, C.A., 2000. Distribution of environmental sensitive trace elements in the Eocene Sorgun coals, Turkey. International Journal of Coal Geology 42, 297–314. Ketris, M.P., Yudovich, Ya.E., 2009. Estimations of Clarkes for Carbonaceous biolithes: world averages for trace element contents in black shales and coals. International Journal of Coal Geology 78, 135–148. Kimura, T., 1998. Relationships between inorganic elements and minerals in coals from the Ashibetsu district, Ishikari coal field, Japan. Fuel Processing Technology 56, 1–19. Kiss, L.T., King, T.N., 1977. The expression of results of coal analysis: the case for brown coals. Fuel 56, 340–341. Kiss, L.T., King, T.N., 1979. Reporting of low-rank coal analysis—the distinction between minerals and inorganics. Fuel 58, 547–549. Kolker, A., Palmer, C., Ruppert, L.F., 2003. USGS Short Course C, Modes of Occurrence of Trace Elements in Coal. (151 slides). Kolker, A., Palmer, C.A., Bragg, L.J., Bunnell, J.E., 2005. Arsenic in coal. USGS Fact Sheet 2005-3152 (4 pp.). Li, Z., Moore, T.A., Weaver, S.D., Finkelman, R.B., 2001. Short communication-Crocoite: an unusual mode of occurrence for lead in coal. International Journal of Coal Geology 45, 289–293. Li, Z., Ward, C.R., Gurba, L.W., 2007. Occurrence of non-mineral inorganic elements in low-rank coal macerals as shown by electron microprobe element mapping techniques. International Journal of Coal Geology 70, 137–149. Liu, G., Yang, P., Peng, Z., Chou, C.-L., 2004. Petrographic and geochemical contrasts and environmentally significant trace elements in marine-influenced coal seams, Yanzhou mining area, China. Journal of Asian Earth Sciences 23, 491–506. Liu, G., Vassilev, S.V., Gao, L., Zheng, L., Peng, Z., 2005. Mineral and chemical composition and some trace element contents in coals and coal ashes from Huaibei coal field, China. Energy Conversion and Management 46, 2001–2009. Mastalerz, M., Drobniak, A., 2007. Arsenic, cadmium, lead, and zinc in the Danville and Springfield coal members (Pennsylvanian) from Indiana. International Journal of Coal Geology 71, 37–53. Miller, B.G., 2011. Clean Coal Engineering Technology. Elsevier Inc.. (696 pp.). Miller, R.N., Given, P.H., 1986. The association of major, minor and trace elements with lignites: 1—Experimental approach and study of a North Dakota lignite. Geochimica et Cosmochimica Acta 50, 2033–2043. Mukherjee, S., Srivastava, S.K., 2005. Trace elements in high-sulfur Assam coals from the makum coalfield in the northeastern region of India. Energy & Fuels 19, 882–891. Mukherjee, K.N., Dutta, N.R., Chandra, D., Pandalai, H.S., Singh, M.P., 1988. Statistical approach to the study of the distribution of trace elements and their organic/inorganic affinity in Lower Gondawana coals of India. International Journal of Coal Geology 10, 99–108. Mukhopadhyay, P.K., Goodarzi, F., Crandlemire, A.L., Gillis, K.S., MacNeil, D.J., Smith, W.D., 1998. Comparison of coal composition and elemental distribution in selected seams of the Sydney and Stellarton Basins, Nova Scotia, Eastern Canada. International Journal of Coal Geology 37, 113–141. Pentari, D., Foscolos, A.E., Perdikatsis, V., 2004. Trace element contents in the Domeniko lignite deposit, Elassona basin, Central Greece. International Journal of Coal Geology 58, 261–268. Poppe, L.J., Paskevich, V.F., Hathaway, J.C., Blackwood, D.S., 2001. A laboratory manual for X-Ray powder diffraction. U. S. Geological Survey Open-File Report 01-041 (88 pp.). Qi, C., Liu, G., Chou, C.L., Zheng, L., 2008. Environmental geochemistry of antimony in Chinese coals. The Science of the Total Environment 389, 225–234. Querol, X., Fernandes-Turiel, J.L., Lopez-Soler, A., 1995. Trace elements in coal and their behaviour during combustion in a large power station. Fuel 74, 331–343. Ren, D., Xu, D., Zhao, F., 2004. A preliminary study on the enrichment mechanism and occurrence of hazardous trace elements in the Tertiary lignite from the Shenbei coalfield, China. International Journal of Coal Geology 57, 187–196. Renton, J.J., Bird, D.S., 1991. Association of coal macerals, sulphur, sulphur species and the iron disulphide minerals in three columns of the Pittsburgh coal. International Journal of Coal Geology 17, 21–50. Rimmer, S.A., 1991. Distributions and associations of selected trace elements in the Lower Kittanning seam, western Pennsylvania, USA. International Journal of Coal Geology 17, 189–212. Ruppert, L., Finkelman, R.B., Boti, E., Milosavljevic, M., Tewalt, S., Simoń, N., Dulong, F., 1996. Origin and significance of high nickel and chromium concentrations in Pliocene lignite of the Kosovo Basin, Serbia. International Journal of Coal Geology 29, 235–258. Schopf, J.M., 1956. A definition of coal: Littleton, Colo., economic geology. Bulletin of the Society of Economic Geologists 51, 521–527. Schweinfurth, S.P., 2003. Coal—a complex natural resource, an overview of factors affecting coal quality and use in the United States. U.S. Geological Survey Circular, 1143 (39 pp.). Shaver, S.A., Hower, J.C., Eble, C.F., McLamb, E.D., Kuers, K., 2006. Trace element geochemistry and surface water chemistry of the Bon Air coal, Franklin County, Cumberland Plateau, southeast Tennessee. International Journal of Coal Geology 67, 47–78.

S.G. Sia, W.H. Abdullah / International Journal of Coal Geology 88 (2011) 179–193 Sia, S.G., Dorani, J., (unpublished). Evaluation of coal resources of the Mukah Coalfield, Sarawak, Malaysia, Minerals and Geoscience Department, Sarawak, GSKL 002/10/ 148, 49pp. Singh, S., Ram, L.C., Masto, R.E., Verma, S.K., 2011. A comparative evaluation of minerals and trace elements in the ashes from lignite, coal refuse, and biomass fired power plants. International Journal of Coal Geology 87, 112–120. Spears, D.A., Tewalt, S.J., 2009. The geochemistry of environmentally important trace elements in UK coals, with special reference to the Parkgate coal in the Yorkshire– Nottinghamshire Coalfield, UK. International Journal of Coal Geology 80, 157–166. Spears, D.A., Zheng, Y., 1999. Geochemistry and origin of elements in some UK coals. International Journal of Coal Geology 38, 161–179. Speight, J.G., 1994. The Chemistry and Technology of Coal, 3rd ed. Marcel Dekker, New York. (642 pp.). Speight, J.G., 2005. Handbook of Coal Analysis. John Wiley & Sons, Inc., Hoboken, New Jersey. (222 pp.). Stach, E., Mackowsky, M.Th., Teichmuller, M., Taylor, G.H., Chandar, D., Teichmuller, R., 1982. Stach's Textbook of coal petrology, 3rd edition. Gebruder Borntraeger, Berlin. (535 pp.). Suárez-Ruiz, I., Flores, D., Marques, M.M., Martinez-Tarazona, M.R., Pis, J., Rubiera, F., 2006. Geochemistry, mineralogy and technological properties of coals from Rio Maior (Portugal) and Peñarroya (Spain) basins. International Journal of Coal Geology 67, 171–190. Sun, R., Liu, G., Zheng, L., Chou, C.-L., 2010. Geochemistry of trace elements in coals from the Zhuji Mine, Huainan Coalfield, Anhui, China. International Journal of Coal Geology 81, 81–96. Swaine, D.J., 1990. Trace Elements in Coal. Butterworths, London. (278 pp.). Swaine, D.J., Goodarzi, F., 1995. Environmental Aspects of Trace Elements in Coal. Kluwer Academic Publishers, Dordrecht. (312 pp.). Tewalt, S.J., Bragg, L.J., Finkelman, R.B., 2001. Mercury in U.S. coal—abundance, distribution, and modes of occurrence. USGS Fact Sheet FS-095-01 (4 pp.). Thornton, I., Farago, M.E., Keegan, T., Nieuwenhuijsen, M.J., Colvile, R.N., Pesch, B., Ranft, U., Miskovic, P., Jakubis, P., EXPASCAN study group, 2003. Environmental impacts, exposure assessment and health effects related to arsenic emissions from a coal-fired power plant in Central Slovakia; the EXPASCAN Study. Arsenic Exposure and Health Effects V, 39–49. Tucker, M.E., 1981. Sedimentary Petrology, an Introduction. Blackwell Scientific Publications, Oxford. (252 pp.). US EPA, 1976. Subtitle D of Resource Conservation and Recovery Act. US Public Law, 1990. Clean Air Act amendments of 1990. Public Law, pp. 101–549 (441 pp.). Valković, V., 1983. Trace Elements in Coal, Volume 1. CRC Press, Inc, Florida. (207 pp.). Vassilev, S.V., Braekman-Danheux, C., 1999. Characterization of refuse-derived char from municipal solid waste 2. Occurrence, abundance and source of trace elements. Fuel Processing Technology 59, 135–161. Vassilev, S.V., Vassileva, C.G., 1996. Occurrence, abundance and origin of minerals in coals and coal ashes. Fuel Processing Technology 48, 85–106. Velde, B., 1992. Introduction to Clay Minerals—Chemistry, Origin, Uses and Environmental Significance. (198 pp.). Visser, W.A., Crew, W.E, 1950 (unpublished), Stratigraphy of Teres–Penipah–Balingian Area, Sarawak, Sarawak Shell Oilfield Limited, GSKL 111/5.

193

Wang, M., Zheng, B., Wang, B., Li, S., Wu, D., Hu, J., 2006. Arsenic concentrations in Chinese coals. The Science of the Total Environment 357, 96–102. Wang, J., Yamada, O., Nakazato, T., Zhang, Z.G., Suzuki, Y., Sakanishi, K., 2008. Statistical analysis of the concentrations of trace elements in a wide diversity of coals and its implications for understanding elemental modes of occurrence. Fuel 87, 2211–2222. Wang, X., Dai, S., Sun, Y., Li, D., Zhang, W., Zhang, Y., Luo, Y., 2011. Modes of occurrence of fluorine in the Late Paleozoic No. 6 coal from the Haerwusu Surface Mine, Inner Mongolia, China. Fuel 90, 248–254. Ward, C.R., 1991. Mineral matter in low-rank coals and associated strata of the Mae Moh Basin, northern Thailand. International Journal of Coal Geology 17, 69–93. Ward, C.R., 1992. Mineral matter in Triassic and Tertiary low-rank coals from South Australia. International Journal of Coal Geology 20, 185–208. Ward, C.R., 2002. Analysis and significance of mineral matter in coal seams. International Journal of Coal Geology 50, 135–168. Ward, C.R., Corcoran, J.F., Saxby, J.D., Read, H.W., 1996. Occurrence of phosphorus minerals in Australian coal seams. International Journal of Coal Geology 30, 185–210. Ward, C.R., Spears, D.A., Booth, C.A., Staton, I., Gurba, L.W., 1999. Mineral matter and trace elements in coals of the Gunnedah Basin, New South Wales, Australia. International Journal of Coal Geology 40, 281–308. Widodo, S., Oschmann, W., Bechtel, A., Sachsenhofer, R.F., Anggayana, K., Puettmann, W., 2010. Distribution of sulfur and pyrite in coal seams from Kutai Basin (East Kalimantan, Indonesia): implications for paleoenvironmental conditions. International Journal of Coal Geology 81, 151–162. Wolfenden, E.B., 1960. The geology and mineral resources of the Lower Rajang Valley and adjoining areas, Sarawak. Memoir, 11. Geological Survey of British Borneo (167p.). Xu, M., Yan, R., Zheng, C., Qiao, Y., Han, J., Sheng, C., 2003. Status of trace element emission in a coal combustion-process: a review. Fuel Processing Technology 85, 215–237. Yang, Z., Zheng, X., Yi, Y., 2006. Investigation of death reason and prevention about arsenic poisoning caused by coal in Xingren, Guizhou. Guandong Trace Elements Science 13 (12), 18–24 (in Chinese). Yudovich,, Ya.E., Ketris, M.P., 2005. Arsenic in coal: a review. International Journal of Coal Geology 61, 141–196. Zeng, R., Zhuang, X., Koukouzas, N., Xu, W., 2005. Characterization of trace elements in sulfur-rich Late Permian coals in the Heshan coal field, Guangxi, South China. International Journal of Coal Geology 61, 87–95. Zhao, Y., Zhang, J., Huang, W., Wang, Z., Li, Y., Song, D., Zhao, F., Zheng, C., 2008. Arsenic emission during combustion of high arsenic coals from Southwestern Guizhou, China. Energy Conversion and Management 49, 615–624. Zharov, Yu, N., Meytov, E.S., Sharova, I.G., et al., 1996. Valuable and Toxic Elements in Russian Row Coals. A Reference Book. (Compilers) Entrails Publ. House, Moscow. (239 pp.). Zheng, B.S., Yu, X.Y., Zhang, J., Zhou, D.X., 1996. Environmental geochemistry of coal and endemic arsenism in southwest Guizhou, P.R. China. 30th International Geological Congress [Abstract], vol. 3, p. 410. Zheng, B., Ding, Z., Huang, R., Zhu, J., Yu, X., Wang, A., Zhou, D., Mao, D., Su, H., 1999. Issues of health and disease relating to coal use in southwestern China. International Journal of Coal Geology 40, 119–132.